This invention relates to monolithic laser/modulator devices and more particularly to a monolithic laser/modulator coupled via a low loss, two dimensional transparent waveguide.
High speed optical molulators are of great interest because of their use in high bit rate optical communication systems. It is necessary that such a modulator be substantially void of wavelength chirping under conditions of high speed modulation, such as above 1 Gbit per second because wavelength chirping will bring about a dispersion of the power and create a noise level that is unacceptable for successful high bit operation. See the article of K. Wakita et al, entitled "Long-Wavelength Waveguide Multiple Quantum Well (MQW) Optical Modulator With 30:1 On/Off Ratio", Electronic Letters, Vol. 22(17), pp. 907-908, Aug. 14, 1986.
It is desired that for improved operation that these optical modulators be monolithically integrated with laser sources as exemplified in the article of Y. Kawamura et al, entitled "Monolithic Integration of InGaAsP/InP DFB Lasers and InGaAs/InAlAs MQW Optical Modulators", Electronic Letters, Vol. 22(5), pp. 242-243, Feb. 27, 1986. However, these monolithic integrated laser modulators involve separate, sequential growth processes and the introduction of unacceptable levels of propagation, absorption and scattering losses.
What is needed is a concurrent as-grown monolithic integration of such optoelectronic devices utilizing means by which these losses can be substantially reduced to levels to provide a commercially acceptable high speed optical laser modulator for high speed modulation. Such integration of active laser sources with modulators is an important capability for the realization of advanced optoelectronics involving on-chip optical communication and, in particular, reducing the levels of absorption loss, and optical scattering and reflection losses at the coupling interface between the integrated active laser source and modulator and, further, providing an integrated waveguide that is transparent at the emission wavelength of the integrated laser source. These features have not been present in such monolithic optoelectronic devices as successfully reproducible capability up to this point in time.
One manner of forming a coupling interface between a passive waveguide and an active medium has been accomplished by evanescently coupling of a transparent passive waveguide formed adjacent and parallel to the waveguide of the active medium. As example of this type of coupling is found in the article of N. K. Dutta et al. entitled, "Integrated External Cavity Laser", Applied Physics Letters, Vol. 49(19), pp. 1227-1219, Nov. 10, 1986. However, such a coupling mechanism has considerable coupling loss and scattering at the transition between the active medium and the transparent waveguide. It is also preferred that the active medium and the transparent waveguide were integrally coupled in co-axial alignment, i.e., are at least co-extensive at the point of coupling.
In recent years, the technique of impurity induced disordering (IID) has been developed as a means for crafting semiconductor structures, which technique may be defined as a process of enhanced rate of interdiffusion of ordered elemental constituents as initially formed in consecutively deposited layers of semiconductor compounds or alloys through the introduction, i.e., diffusion, of an impurity into the layers. The utility of IID, as discussed in K. Meehan et al, "Disorder of an Al.sub.x Ga.sub.1-x As-GaAs Superlattice by Donor Diffusion", Applied Physics Letters, Vol. 45(5) pp. 649-651, Sept. 1, 1984 and in U.S. Pat. No. 4,639,275, has been demonstrated in the fabrication of buried heterostructure lasers, as per the article of R. L. Thornton et al entitled, "High Efficient, Long Lived AlGaAs Lasers Fabricated by Silicon Impurity Induced Disordering", Applied Physics Letters, Vol. 49(3), pp. 133-134, July 21, 1966. Also, it has been previously shown that IID can be employed to fabricate a planar, transparent one dimensional passive waveguide for a window laser as demonstrated in the article of R. L. Thornton et al entitled, "High Power (2.1)W) 10-Stripe AlGaAs Laser arrays With Si Disordered facet Windows", Applied Physics Letters, Vol. 49(23), pp. 1572-1574, Dec. 8, 1986. However, as the laser emission aperture is made narrower, as is desirable for a low threshold buried heterostructure laser, the diffraction losses in the waveguide window will increase. Thus, what is necessary is to either cleave the device so that the window region is only a few microns long in order to best compromise low threshold against diffraction losses or, alternatively, fabricate an efficient buried two dimensional passive, transparent waveguide in the window region. The former is difficult to manifest in a reproducible manner, leaving the latter as the best alternative for improving the propagation efficiency and coupling of buried transparent waveguides.
There has been work studying the optical waveguide properties of two dimensional passive waveguides fabricated by IID as, for example, the article of F. Julien et al, "Impurity-Induced Disorder-Delineated Optical Waveguides in GaAs-AlGaAs Superlattices", Applied Physics Letters, Vol. 50(14), pp. 866-868, Apr. 6, 1987. However, these waveguides exhibit high propagation loss at the emission wavelength of a laser fabricated in the same set of epitaxial layers.
A principal object of this invention is the provision of a monolithic laser/modulator that has a modulator section and an amplifying section coupled by a low loss, two dimensional transparent waveguide, that provides low propagation loss, low scattering loss and low optical absorption at the gain wavelength of the active medium providing output light pulses significantly narrower than 100ps.